Method for firing graded glass tubular seals



Jan. 17, 195@ .1. c. PROKOPEC 2,494,582

METHOD FOR FIRING GRADED GLASS TUBULAR SEALS Filed Aug. 9, 1944 linhcnmr JAMES ("A AMA; fifialro ec Qtturnep fatented Jan. 17 1956 as er METHOD FOR FIRING GRADED GLASS TUBULAR SEALS James Charles Prokopec, Corning, N. Y., assignor to Corning Glass Works, Corning, N. Y., a corporation of New York Application August 9, 1944, Serial No. 548,740

This invention relates to methods of making graded glass seals, particularly tubular graded seals, for use in joining glasses having widely different thermal expansion coefficients. As is well known, such seals are composed of a series of glasses joined together by fusion, the thermal expansion coefficient of each glass differing from that of the adjoining glasses by an amount insufiicient to cause the establishment of breaking stresses and varying progressively from the lowest to the highest in the series.

Tubular graded seals are usually made by joining short lengths of glass tubing having the desired range of expansion coefficients by careful fusion and manipulation in a fiame. This is a slow laborious process requiring great manual skill and it requires a complete series of glasses having the characteristics necessary to bridge the thermal expansion difference desired. Due to the scarcity of glasses to provide steps between quartz glass or fused silica and commercial glasses of the lowest thermal expansion coefiicient, graded seals for joining fused silica and ordinary glasses are not readily made by this method.

Tubular graded seals have also been made by firing porous glass tubing of high silica content impregnated with salts or compounds of boron and/or alkali metals in a tubular gradient furnace having a progressively varying temperature throughout its length. Such gradient furnaces cannot be constructed or controlled so as to provide temperature gradients of 500 C. or more through the short distance of about one inch or less as required for short graded seals.

Graded seals are highly desirable which are composed of two pulverized glasses intimately mixed in progressively varying proportions and molded into shape by a method such as is set forth in Patent 2,390,354 issued December 4, 1945, to Charles W. Clapp and assigned to the assignee of this invention. However, such seals heretofore could not successfully be fired to produce a uniformly symmetrical vitreous tube, due to the relatively large shrinkage of the powdered glass on sintering and to the wide range of softening temperatures between the two glasses which resulted in severe distortion'despite the use of gradient heating.

The primary object of this object of this invention is to provide a method of making graded seals rapidly and economically.

Another object is to make uniformly symmetrical short tubular graded seals.

A further object is to make tubular graded seals which will bridge the expansion difference between quartz glass or fused silica and ordinary low expansion glasses or the expansion difference between glasses of low expansion and those of high expansion.

Another object is to fire a molded pulverized 3 Claims. (01. 4982) "designated A and B respectively.

glass article having graded expansion characteristics and softening temperatures. I

Still another object is to fire a tubular graded seal of molded pulverized glass without distortion thereof. p

The above and other objects may be accomplished by practicing my invention, which in its broader aspect comprises exposing the portion of the molded article having the highest softening temperature to a source of radiant heat having a substantially uniform temperature in the neighborhood of said softening temperature while partially shielding the lower-softening portions of the article and extracting heat therefrom in proportion to their softness.

In order that the invention may better be understood, reference is had to the accompanying drawing in which:

Fig. 1 is a sectional view of a tube molded from a series of mixtures of two pulverized glasses Fig. 2 is a sectional view of the tube shown in Fig. 1 after being fired near the softening temperature of glass B.

Fig. 3 is a plan view of a heat-protective apparatus containing tubular graded seals of molded pulverized glass to be differentially fired in accordance with my invention.

Fig. 4 is a sectional view on the line 4-4 of Fig. 3, showing the apparatus within a radiant electric furnace.

Fig. 5 is an enlarged fragmentary sectional view of the apparatus of Figs. 3 and 4.

Fig. 6 is an enlarged fragmentary sectional view of a modification of the apparatus shown in-Fig. 5.

In the various figures the same marks of reference are used to designate the same parts.

My invention is applicable to molded glass articles having progressively varying softening temperatures and composed of any desired pair or combination of pulverized glasses having different thermal expansion coefiicients and the following description refers, by way of example only, to tubes composed of pulverized mixtures of glasses A and B which I have used and which have the following physical properties:

GlassA I GlassB Expansion Coefficient 8X10- Softemng Temperature 1450" G.

ing progressively from glass A alone at one end of the tube to glass 5B alone at the other end of the tube.

In Figs. 3, 4, and 5 a plurality of short tubula graded seals in composed of molded pulverized glass are supported on a :plate 1-,! composed of a heat conducting material having a high heat,,.

capacity such as metal, and preferably a nonscaling metal such as the alloy known as Alleghany 55. ceramic refractory material and provided with holes slightly larger than :the tubular graded seals is located above the plate H so that the tubular graded seals lflstand loosely in the holes. The thickness of the slab I2 is preferably slightly 'less than-thelength of the graded seals i6 sothat the uppermost ends of the tubes will project somewhat above the surface of the slab l2. On the-under side of the slab 12 the holes preferably are countersunk or enlarged so as to provide a 'heat radiating and convecting space M around thelower end of each graded seal. The depth to which the space I 4 is countersunk is normally about one-half thefllength'of the tube Hi, but if the softerend of the tube is made proportionately longerthanthe harder end of the tube, as .is sometimes thecase, the depth of the space M should .be increased The holes may, in some instances, be provided with a straight bore as shown in Fig. 6 when the length of the tube is .Very short. Wihhinthe tubular seals H3 preferably are placed small cylinders of insulating refractory .material, which are preferably somewhat shorter than the tubular seals [0 and slightly smaller in diameter. If desired, the cylinders 15 may be omitted ,from tubular seals having an :inside diameter ,as small as one-half inch or less. The plate ll bearing the tube H] is supported ,on a :base slab 46 of insulating ,ceramic refractory material provided with upstanding ,side walls -l,:'| which support the slab I 2, the under side of {the slab I2 beingpreferably contact or close proximity with the plate H.

In carrying :the invention into practice tubular graded seals, composed and molded from mixt r 'o pulv ri d las a i dicat d in F are subjected to a preliminary firing at a temperature near the softening temperature of glass 13 to eliminate organic matter. The time and temperature should be just sufficient to cause the ipper end-of the tubular seals {to partially sinter, the tube-being in such position that its softest portion is uppermost. ',such treatment results i-naslight shrinkage. of the sof-ter end of the tube as shown in Fig. 2.

Tubular-graded seals of the type shown in Fig. ,2, after a preliminary heat-treatment, are placed on ,the metal plate :1 l and ,baseslab IS with their shrunken pr setter ,end incontact with the plate 1450" C. and the graded seals should be thus heated for about fifteen m-inutes. Combinations of other glass compositions having expansion .coeflicients and softening temperatures different from those of glasses A and B will require diiferent times and/or temperatures, but

A slab l2 composed of an insulatin '4 in general the final firing temperature should be near the softening gtemperature of ;the uppermost end of the tube orthe'end composed of the glass mixture having the highest softening temperature.

During the final firing step, the uppermost :end.of the molded glass tube I0 is exposed to the full radiant energy of the heat source, but the j lowe'rmost end-and the intermediate portions of thetube I0 ,are shielded from the direct radiation of the furnace by'the refractory slab l2 and cylinder 4-5. Although the slab l2 and the cylinder,.l 5 tend to soak up heat and become highly heated throughout, the time required for the .sinteringof the uppermostendpfthe tube 10 is .insufiicient for them to attain the maximum -temperaturethroughout. Consequently a constantly decreasin temperature gradient .will exist between the topand bottomoftheslab 1.2 and likewise of the cylinder 15 and radiation therefrom will subjectthe tube 1.0 to asimilar temperature gradient which {decreases towards the lowermost end of .the tube 1-0.

have found that in order to,prevent the lowermost or softer end of-thetube ,H] from thus becoming overheatedduring the time necessary to sinter the ppermost or harder end of the tube til, ;it is necessaryto provide ;t-he plate H which acts to conduct same-pf the excess heat from the lowermost endiof :the tube .10 W iQ iS in contact therewith. -Fai lure to make such ,a

.rproyision results in fusionand collapse of the lowermost end .of the tube 1 I 0.

I have further 'found;that .1the intermediate ,or .central-portionof the tube all) .may also become overheated insthe time required for sintering the upper endthereofunless theradiatingspace [A is provided whereby .excess heat from the intermediate-portion ofthel-tube'z lfl is radiated and .con- :vected' to the plate V M and :thereby conducted away. The bottom slab ==1 6 and-side walls .11

serve :to protect the plate 11 from becoming :too highly located from below and thereby to ensure its 'efiicient functioning.

Thus it =will"be seen that 'in order to subject all portions of the tube lll "to-the temperature and time necessary for its propersintering, the harder end'isheated at'ornear'its softening temperature while'heat is extracted from thesofter portions in proportion to their softness.

It will be apparent that the temperature gradient towhich the intermediate or central portion and the lowermost end of'the tube Ill are subjected during the time necessary to sinter its uppermost end can be varied somewhat by varying the thickness and/or'the composition of the slab l2 and the plate H. Also, the temperature and time of treatment may be varied. For example, good results ma y beobtained'by subjecting the tubestfl -in their-protective apparatus to a higher temperature for a shorter time, provided the harder uppermost endsof the tubes I0 do -not projectabove the slab 42. Such variations are contemplated by meand are intended tube within the scope of the inventionas claimed.

Morecver,-the preliminary firing step may, if desired, be omittedand an'unsintered tube such as -is shown in Fig. 1 may be placed in the-apparatus shown in Figs. 3; 4, 5 and 6, and fired, as described above. However, such a procedure might result in undue breakage of the relatively weak unfi-red tubes in handling and also may not completely eliminate organic material used in bonding and molding the pulverized glass.

I claim:

1. The method of firing a tube composed; of molded powdered glass and varying in composition from a high softening temperature glass at one end of the tube to a substantiallyrlower softening temperature glass at the other end, which includes exposing the portion of the tube having the highest softening temperature to-a source of heat external of the tube and having a temperature in the neighborhood of said highest softening temperature while substantially preventing heat from reaching the lower softening portions of the tube except by conduction through the tube. p

2. The method of firing a tube composed 0f molded powdered glass and varying in composi-l tion from a high softening temperature glass at one end of the tube to a substantially lower softening temperature glass at the other end, which includes exposing the portion of the tube having the highest softening temperature to a source of heat external of the tube and having a temperature in the neighborhood of said highest softening temperature while shielding the lower softening portions of the tube from direct heat from said source but indirectly heating them by conduction through the tube and conducting heat away from the lowest softening portion.

3. The method of flring a tube composed of m'olded powdered glass and varying in composition from a high softening temperature glass at one end of the tube to a substantially lower softening temperature glass at the other end, which includes heating the tube to a temperature and for a time to at least partially sinter the portion having the lowest softening temperature, and thereafter exposing the portion of the tube having the highest softening temperature to a source of heat external of the tube and having a temperature in the neighborhood of said highest softening temperature while shielding the lower softening portions of the tube from direct heat from said source but indirectly heating them by conduction through the tube and conducting heat away from the lowest softening portion.

JAMES CHARLES PROKOPEC.

REFERENCES CITED- The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,014,757 Keyes et al Jan. 16, 1912 1,173,688 Thompson Feb. 29, 1916 1,960,121 Moulton May 22, 1934 2,026,370 Winkler Dec. 31, 1935 

